Andy's ZX ULA is able to do that and offers much more like clock doubling. Unfortunately is not available now. Anyway not fun to solder 10 or more wires instead of now only 3 (GND,5V,Video In) and board wouldn't be so tiny.

Wonderful improvement! Now, how about a frame buffer to hold the screen until display can be executed?

By Pokémon:

I made a study about that earlier already and have a solution in my mind but in the moment no chance to realize. Anyway would be larger in size and more expensive. I have another solution in my mind which may come in summer. Maybe.